Many computer peripheral devices, such as photocopiers, printers and document scanners, rely on transport of a sheet, or sequence of sheets, of paper through the peripheral device to transfer an image to, or to pick up an image from, the sheet. This sheet transport often requires precise measurement of the position of the sheet by a sensing mechanism. Measurement of position of the sheet should be performed non-invasively so that the surface and translated position of the sheet, and any image thereon, are not contacted, disturbed or altered. Precise positioning by optical means may require use of a CCD sensor or an array of such sensors, and the distance from sensor to paper surface must be known as accurately as possible to control image size or magnification and image focus. The sheet may move as much as 1000 .mu.m in a direction perpendicular to the plane of the sheet, and the optical system associated with the sensor must compensate for this motion. Conventional optics, as illustrated in FIG. 1, provides a relatively large cone angle 2.theta. for the light at the optical elements 11, and this angle will change the apparent image size by a substantial amount with a shift in position of the sheet 13 toward or away from the optical elements 11 or the sensor 15.
Nopper, in U.S. Pat. No. 3,919,560, discloses method and apparatus for monitoring the present position of a continuously moving web of material, using photoelectric scanning of the web for one or more of a sequence of index marks that are placed on the web margin before the web is monitored. This invasive approach, requiring placement of index marks beforehand, relies upon artificial markings and requires use of very closely spaced markings in order to assure web position accuracy good to within a few microns. Further, if the photoelectric scan means or the line of index marks move laterally relative to one another, the photoelectric means may not sense the passing of some or all of the index marks as the web moves.
In U.S. Pat. No. 4,373,816, Laib discloses use of a collimated and periodically chopped light beam that is reflected at an angle from a surface of an object, such as a piece of lumber, whose thickness is to be measured. The amount of lateral displacement of the reflected light beam determines the object thickness.
Use of a light sensor to receive light transmitted through a thin moving web of material such as fabric is disclosed in U.S. Pat. No. 4,683,380, issued to Shipkowski et al. Two adjacent photodiodes receive the transmitted light, and a perforation in the web will first manifest its presence as an abrupt difference in light received by the two diodes.
What is needed is an approach that allows determination of the distance of substrate movement in a given direction, where the approach: (1) is not sensitive to movement of the substrate in a direction that is perpendicular to the given direction; (2) is non-invasive and does not contact or alter the surface of the sheet or of any text or graphics contained on the sheet; (3) compensates for the presence of signal noise and other perturbations; (4) does not alter or affect servo response or substrate position in the given direction; (5) is flexible and allows improvements of accuracy with improvements in light measurement technology; and (6) does not require that extraneous features (reference marks, tractor feed holes, etc) be added to the substrate as an encoder strip.